I had a very similar problem. I ended up having to go to the actual Gravatar website- https://en.gravatar.com/ & load my old Gravatar from there. Have you tried that? If you haven’t been there before your log in info is the same as for your WP account. Sometimes you also have to erase the cookies from your browser in order for it to work. Don’t ask me why….it’s just what I was told & it worked. Hope that helps 🙂